Without this purposed modifications, it can be done of course, but with manually editing all the descriptions and definitions of setters and getters, and on the model you would still be saying the setters are void instead of the actual type. By doing this modifications manually it kills the purpose of the add getter/setter shortcut I think.

Let's note you can create automatically your getter/setter as you expect through a plug-out rather than by hand.

Applied to an attribute or an relation this plug-out creates/updates the getter/setter (perhaps using stereotype <<setter>>/<<getter>> on the operation to easily distinguish them from the other operations, and also using a user property to save the name of the associated attribute/relation) , applied on a container (class view / package) this plug-out process recursively to update already created setter/getter including to remove the ones whose attribute/relation do not exist anymore (I don't think it is useful to have a getter/setter for all attributes/relations)